[AMRadio] HOMEBREW


Brett gazdzinski Brett.gazdzinski at mci.com
Wed Sep 29 19:16:29 EDT 2004


For many tubes, it does not seem to be a problem.
813's seem to modulate great with a choke in the screen circuit to allow
self modulation. The 4-125/250/400 tubes do also.

My 813 pair can hit incredible positive peaks, 2500 watts plus.
I did not do the trapezoid test, but as far as audible differences
between the push pull 812 rig, there are none that I can tell.
In truth, the 813 rig sounds a little cleaner because the mod deck is AB1
and has lots of extra power.

The class B 811a mod deck does not sound quite as clean,
even though I drive it with a solid state amp.
I don't think you will ever see a class B push pull triode hi fi amp...

But for ham work, that's nit picking...

I retired the push pull parallel 100th mod deck.
It looks way cool, 4 glowing tubes behind glass, but sounds
quite poor in comparison to the 4x150a deck.

I have not had the heart to remove it from the audio rack
and take it apart.

Two 100th tubes in class B is not bad, but four is impossible,
the stiff and erratic loads to the audio driver cause all sorts
of distortion.
People don't report it sounds bad, but it does to me in the mod
monitor.

Best bet for a transmitter seems to be push pull triodes and a
hi fi AB1 modulator.

When I get the quad of KT90's mod deck built, I can try
it on the 812a RF deck.


On a side note, the 4x150a/4cx250b tube makes a great modulator.
The great things about those tubes are:

Low screen voltage, 350 volts that is easy to electronically regulate,

AB1, NO driving power needed, just voltage,

You can variac the plate voltage between 1000 volts to 2000 volts,
changing the impedance and power a lot, without changing the
screen voltage or bias voltage a bit!

They will put out 600 watts of clean audio.

Brett
N2DTS


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of John Coleman
Sent: Wednesday, September 29, 2004 1:03 PM
To: AMRadio
Subject: [AMRadio] HOMEBREW


I prefer triodes because they tend to have linear plate modulation
characteristics with out a lot of other circuitry.  Tetrodes, on the other
hand, not only require screen circuitry, but require a lot of audio level
screen compensation in order to reach the same characteristics of the
triode.  But I'm very critical on passing the trapezoid pattern test.

 

I wrote an article similar to this point at
http://www.qsl.net/wa5bxo/pptriodes/pptriodes.htm

 

It is some what under construction still.  So I have not made a link from
the home page http://www.qsl.net/wa5bxo

 

I was trying to clear up some things that or either not always clear in
other books or not mentioned at all.  Hope this helps someone.

 

73

John, WA5BXO

 

 

-----Original Message-----

From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Brett gazdzinski

Sent: Wednesday, September 29, 2004 8:15 AM

To: 'Discussion of AM Radio'

Subject: RE: [AMRadio] Home Brew

 

It sounds simple, and the RF stuff is simple, but it's the control and
metering that adds complexity.

 

Step start, push to talk, grid current and voltage metering, plate or
cathode current and plate voltage meters, plate voltage control (variac or
power trans primary taps),filiment voltage control and metering, protective
bias plus grid leak bias, etc.

 

But overall, it is quite simple, one step at a time...

 

I like building a small deck just for the rf, and do the control on a
separate deck, with lots of meters and knobs to twist. Big wire wound pots
work great for grid leak resistors.

 

Add in control grids and it gets more complex, with sequence starting, more
power supplies and metering, overload circuits, etc.

 

Its loads of fun to figure out what will work with what

parts you have or can get.

 

On the subject of meters (I use a lot of them), I use  the cheap radio shack
meters, they used to make 0-15 volts, 0-500 ma, and 0 to 1ma meters that
took a very small hole in the panel. I just make the scales indicate
whatever I want, 0-15 volts can be 0 to 150 volts, 0 to 1500 volts, 1.5
amps, 150 ma, etc. The scales come off, they are held on with glue, and you
can make up your own numbers or transfer them from one meter to another, as
I think they only sell the 0-15 volt meter now. I should use a computer
program to make up real scales.

 

So you can have a load of meters, all the same, cheap cost, small panel
holes, and modern looking. Shunts will give any current you want, voltage
dividers give whatever voltage you want.

 

Black painted panels with P touch labels looks nice, and don't count on
remembering what meter does what, or where its supposed to read after 10
years or more, or the next guy who gets the equipment! You can mark the
correct voltage/current with a red felt tip marker.

 

A pair of 813's takes how much grid drive? Screen current? Where did you run
the plate current on the 4d32 rig? Are the 4x150a filaments 6 volts, or 6.3?

 

Is this stuff fun or what?

 

Brett

N2DTS

 

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